Patients with the Smith-Lemli-Opitz syndrome have a cholesterol deficiency because there is a block in the synthesis of cholesterol. Clinically, they have microcephaly, mental retardation and hypotonicity, along with very low plasma cholesterol levels and high concentrations of 7- dehydrocholesterol. The studies involve feeding dietary cholesterol to alleviate the deficiency and to block biosynthesis of 7- dehydrocholesterol.